The REST endpoint is /_settings (to update all indices) or
{index}/_settings to update one (or more) indices settings.
The body of the request includes the updated settings, for example:

PUT /twitter/_settings
{
"index" : {
"number_of_replicas" : 2
}
}

The list of per-index settings which can be updated dynamically on live
indices can be found in Index Modules.
To preserve existing settings from being updated, the preserve_existing
request parameter can be set to true.

For example, the update settings API can be used to dynamically change
the index from being more performant for bulk indexing, and then move it
to more real time indexing state. Before the bulk indexing is started,
use:

PUT /twitter/_settings
{
"index" : {
"refresh_interval" : "-1"
}
}

(Another optimization option is to start the index without any replicas,
and only later adding them, but that really depends on the use case).

Then, once bulk indexing is done, the settings can be updated (back to
the defaults for example):